Screening mammography is an established method for detecting early signs of breast cancer in otherwise asymptomatic women. The American College of Radiology recommends annual mammography for women over forty. Women undergo an X-ray exam in which X-ray films of the breast are exposed and then developed for later review. A radiologist reads the films and assesses the likelihood of the presence of signs of breast cancer for the case. If a suspicious finding is present, the woman will typically be invited for additional, more detailed diagnostic X-ray exams, followed by ultrasonic exams, and finally, biopsy.
In a typical screening exam in the United States of America, four X-rays of a women's breast are obtained. Two mammographic views are obtained for each breast: a cranio-caudal view is obtained by positioning the X-ray film horizontally under the compressed breast, and a medio-lateral oblique view is obtained by positioning the X-ray film in a plane that is approximately orthogonal to the left-right axis. In some situations, more or fewer X-ray views may be obtained. For example, women with large breasts, implants, or unilateral mastectomies may have more or fewer X-ray films to cover all breast tissue.
As shown in FIG. 1, the four views are typically labeled LCC (Left Cranio-Caudal), RCC (Right Cranio-Caudal), LMLO (Left Medio-Lateral Oblique) and RMLO (Right Medio-Lateral Oblique). Knowledge of the laterality (i.e., Left/Right) and type of mammographic view (i.e., CC or MLO) is critical for the review process. Within each pair, the left image corresponds to the right laterality of the patient, and the right image corresponds to the left laterality of the patient.
Each mammography site, and often each individual radiologist, has established procedures and preferences for the positioning of the four views/films with respect to each other on the light box. For example, the LCC and RCC views may be positioned next to each other, with the chest wall on both views in close proximity of each other, and the LMLO and RMLO views may be positioned underneath. Similarly, the LCC-RCC and LMLO-RMLO pairs may be positioned beside each other. In addition, prior exams (that is, previous years) of the same woman might also be positioned on the same light box for simultaneous review and comparison in well-defined positions with respect to each other and the current mammographic films.
Radiologists follow a well-established and repeatable routine in reviewing mammograms, with and without a magnifying glass, which is why a repeatable positioning of the films on the light box is important and indispensable.
The diagnosis procedure often involves the use of Computer-assisted detection (CAD) which is part of a method to increase the sensitivity of the mammographic screening process. After the X-ray films have been developed, but before they have been reviewed by a radiologist, they are digitized by a high-resolution digitizer. The digitized images are then processed by an algorithm server using computer-aided detection algorithms to automatically identify regions that may be consistent with signs of breast cancer. When the radiologist reviews the films, and after she/he has arrived at a diagnosis, a computer screen or printout is used to present the results of the algorithmic detection process to the radiologist in the form of marks on the mammographic images. The radiologist can use some of these marks to re-review the mammographic films, and in a some instances, change his/her opinion/diagnosis. In this manner, the computer algorithms in a CAD system can serve as a “second reader” to help increase the sensitivity of the mammographic review process.
The computer algorithms typically accept the four views of a screening mammography exam as a group. Although it is possible for the algorithms to process each of the digitized images without knowledge about their laterality or the type of mammographic view, the performance of the algorithms might improve if this knowledge is available to the algorithms, i.e. if the laterality and type of mammographic view is assigned to each digitized image.
The marks for suspicious regions can be placed on a reduced version of the digitized mammographic film. If so, it is advantageous if the reduced version of the digitized mammographic films on which the marks for suspicious regions are presented to the radiologist are arranged in exactly the same way as their film counterparts are arranged on the light box. In this manner, the radiologist can readily navigate between the films, the digitized versions, and the CAD marks to quickly re-review regions on the film that had been flagged by the CAD algorithms as being suspicious.
However, the assignment of laterality or type of mammographic view is not encoded in the mammographic films in computer-readable form. As a result, typical CAD systems require that the four views be fed into the digitizer in a pre-defined order, and in a pre-defined orientation. The pre-ordering and pre-orientation process is laborious, time-consuming, and error-prone.
U.S. patent application Ser. No. 09/721,347, incorporated herein by reference, describes a method wherein lead markers identifying the laterality and type of mammographic view are typically positioned on the X-ray film prior to exposure of the breast and the film to X-ray. These markers appear as text (for example, “RCC”) on the films, and therefore could be recognized by a form of template matching. However, template matching algorithms require the use of unique and well-characterized lead markers.
U.S. Pat. No. 5,748,173 discloses methods that allow correlation of films on a light box with digitized images in a computer database based on identification information encoded in the film on the light box. The identification methods include magnetic encoding, barcodes or character recognition on CCD camera images of patient information printed on the film. However, this method suffers from the complexity of affixing magnetic or bar code information on the film, or from the complexity of positioning one or more CCD cameras such that they will reliably image patient information from the films on a light box, and from the complexity of character recognition and text understanding based on such images obtained with CCD cameras.
In a commercially available system from icad, Inc. (Nashua, N.H.), a barcode label with patient identification, laterality and type of mammographic view is affixed to each film prior to digitization and CAD processing. As each film is digitized, a barcode reader obtains laterality and type of mammographic view from the barcode label, and assigns this to the digitized image. During the reading process, each film on the light box is correlated with its digitized counterpart using a barcode reader as described in U.S. Pat. No. 5,748,173. A drawback of this method lies in the need to affix a special barcode label to each film, a laborious process comparable to ordering and orienting films prior to digitization.
Therefore, a need exists to overcome the drawbacks and disadvantages of existing and known systems. More particularly, there exists a need for alternative methods to those described in the prior art for assigning laterality and type of mammographic view to digitized images of X-ray film mammograms.